A dielectric barrier discharge type low pressure discharge lamp (EEFL) is known, which is provided with electrodes on an external surface of a tubular glass lamp vessel, as described in the Japanese official gazette of the utility model laid open No. 61-126559, for example. The configuration of the conventional low pressure discharge lamp is shown in FIG. 3.
In FIG. 3, a low pressure discharge lamp 15 has a tubular glass lamp vessel 10, both ends of which are sealed. An ionizable discharge medium 50 such as rare gas or a mixed gas of mercury and rare gas, is enclosed inside the tubular glass lamp vessel 10. A phosphor layer 60 etc. is formed on the inner surface of the tubular glass lamp vessel, if necessary. External electrodes 25, 26 are provided on the outer surface of both ends of the tubular glass lamp vessel. The external electrodes 25, 26 are made of electrical conductive material layers 35, 36 such as, for example, a metal foil attached on the glass surface through an adhesive layer, such as in an aluminum tape electrode. Electricity feeding members 75, 76 are attached on the external electrodes 25, 26, while lead wires 81,82 are attached on the electricity feeding members 75, 76.
A low pressure discharge lamp 15 with such construction has an advantage that the consumption of electrodes is avoided and the life is long, because the electrodes are not provided inside the glass lamp vessel 10.
However, when the electrical conductive material layer 35, 36 are formed by a metal foil such as an aluminum tape electrode, a high lamp voltage must be applied to the conductive material layer 35, 36 due to an insufficient contact with the tubular glass lamp vessel 10 and to a high electric resistance of the conductive material layer 35, 36 themselves. To solve the problem it is proposed to form the external electrodes with a solder layer using an ultrasonic solder dipping method. A lamp voltage becomes lower in such an external electrode type lamp having a metal layer such as a solder electrode directly formed on a glass surface than an external electrode type lamp having a metal foil attached on an external surface of the glass lamp vessel through an adhesive layer, such as an aluminum tape electrode since the electric resistance of the electrodes themselves can be small due to their sufficiently thin thickness. Therefore, there is also an advantage that circuit design of an inverter for generating high voltage high frequency electric power becomes easier.
However, a solder electrode has a low heat capacity because the thickness is about one twentieth as thin as that of the aluminum tape electrode. For this reason, the solder electrode tends to exhibit partially uneven electrode temperature distribution compared with aluminum tape electrode. For example, in the conventional example shown in FIG. 3, where the electricity feeding members 75, 76 are provided only in the vicinity of a central portion, the temperature in the central portion of the electrode tends to decrease by heat dissipation, while the temperature on the both ends of the electrode, where electricity feeding members are not arranged, tends to become high. Therefore, there was a problem that the electrode temperature became locally high at the vicinity of the ends of the electrodes, and thus the glass material is molten to form a hole, which enables the lamp to be lit.
One of the objects of the present invention is to solve such problems, and to provide a low pressure discharge lamp in which adverse effects due to the local temperature rise in the external electrode surfaces formed by a solder layer are reduced.